Infantile spasms (IS) are a particular form of seizure, usually confined to infancy and often associated with mental retardation. Where IS are associated with hypsarrhythmia (a chaotic brain wave pattern) on electroencephalogram (EEG) and developmental arrest, the term West syndrome is used. IS are often symptomatic of a heterogeneous group of aetiologies such as structural brain malformations or brain injury. A rare subgroup of IS is due to genetic factors (idiopathic IS) and generally has a better prognosis than the symptomatic group. A subgroup of IS is X-linked IS which carries a poor prognosis (ISSX, MIM 308350). The ISSX family of Bruyere et al. initially defined the candidate ISSX gene region to ˜7 Mb, between DXS1226 and AHC (Adrenal Hypoplasia Congenital).